What is a sinkhole? What causes one? - Kathrine, 12, Calgary, Canada

Sinkholes can be scary to think about. They don’t happen too often, but when they do, they can take people by surprise. The solid ground disappears, and a hole suddenly appears.

It might seem like sinkholes appear out of nowhere. But they actually need specific conditions to form.

To have a sinkhole, you first must have a cave.

“You can think of a sinkhole as the end of the life cycle of a cave,” Kurtis Wilkie explained. He teaches Geology at Washington State University. He is very interested in how Earth’s features form over long periods of time.

A lot happens underground that we can’t see. Dirt and rock layers lie beneath our feet. Water flows around them, shifting and moving these layers.

With the right type of rock, enough water, and a lot of time, a cave can form.

Wilkie said caves often occur in rock called limestone. Limestone is made mostly of calcium carbonate (the same substance that makes up seashells!).

Limestone isn’t a very strong type of rock. It’s full of tiny cracks. They’re hard to see, but big enough for water to run through. Lots of contact with water can make those gaps get bigger. Over time, the limestone dissolves and breaks apart. This process is called erosion.

As the rock dissolves, empty space gets left behind. Eventually, that space gets bigger and bigger until a cave forms. This happens extremely slowly, much longer than any human’s lifetime.

“We’re talking not just thousands of years, maybe millions of years. It’s not as if you start the process now and then 10 years or 100 years from now you have a cave. It takes a very long time,” Wilkie said.

Most caves remain caves. But if water continues to interact with limestone, it can keep slowly eroding. The cave’s roof can become too weak to hold the heavy ground above it. If the roof collapses, the ground above it falls through. That’s how a sinkhole happens, and part of the cave comes to an end.

A sinkhole is the end of a cave’s life—but not every cave’s life. Most caves don’t ever collapse or turn into sinkholes. A sinkhole only happens if the cave’s roof becomes too thin and unsupported. Humans can cause sinkholes to happen more than they would naturally by pumping water from underground, reducing support for the ground above.

Sinkholes happen more in some places than others. You might hear about sinkholes in Florida, an area with lots of limestone. But here in Washington State, where I live, other types of rock abound. So sinkholes are very rare.

The odds of the ground collapsing beneath you are very small. You’re much more likely to get to visit a cave someday.

And if you do, you can look up at its walls and remember the forces that shaped it. All it takes is a special rock, a lot of water, and plenty of time.